1. Field of the Invention
This invention relates to spinal fixation.
2. Description of the Prior Art
The normal human spine is a segmented column of vertebrae, coupled together by intervertebral joints of two types; viz. anteriorly by intervertebral discs and posteriorly by zygoapophyseal gliding synovial joints.
Spinal deformity occurs when the normal alignment of the vertebrae is upset or disturbed by altered muscle or nerve function or, more rarely, by abnormal bony structure. Once deformity has been established, secondary changes take place which maintain the abnormality and prevent its return to normal. When the deformity is sufficiently profound, it may be associated with an instability such that the deformity will tend to increase with the axial loading by gravity in spite of the removal of the causative factor.
Under these circumstances it is often necessary to fix together the vertebral levels, after having returned them towards their normal position if safely possible. For this purpose, it is known from, for example British Patent Specification Nos. 1241292; 2051581 and 2131300; U.S. Pat. No. 4,611,581 and European Patent Specification No. 0128058, to provide fixation implants, each of which comprises an elongate member and a plurality of devices for anchoring the elongate member to respective vertebrae. British Patent Specification No. 1241292 discloses an implant which is for use posteriorly and in which the elongate member is a rigid channel member formed at intervals therealong with bores for receiving respective bone hooks having threaded shanks for insertion through the bores to cooperate with nuts which urge the vertebrae towards the channel member. This system has a number of disadvantages, for example that the hooks, and thus their vertebrae, cannot be adjusted relative to each other along the member after fixation and that the hooks cannot be adjusted around the member. The implant disclosed in U.S. Pat. No. 4,611,581 has similar disadvantages, except that adjustment of its bone screws, and thus their vertebrae, relative to each other along the member after fixation is possible, but to an extent limited by bridging portions among slots along the plate-form elongate member. Another implant, shown in FIG. 1 of British Patent Specification No. 2051581, is for use posteriorly and comprises as its elongate member a rigid rod having two hook-like devices connected to respective ends thereof which hook respectively over and under respective vertebrae. With the implant in position on the spine, the devices are adjustable only away from each other. Another of these implants, also shown in that Specification, is again for use posteriorly and comprises as its elongate member a rigid screw-threaded rod having hook-like devices connected thereto which hook respectively over and under respective vertebrae. Each device includes a channel-form base portion which receives the threaded rod and the channel of which incorporates a bore which houses a sleeve encircling the rod. The external diameter of the sleeve is greater than the width of the mouth of the channel, so that the sleeve and thus the rod cannot be displaced radially out of the channel. However, the sleeve can be removed longitudinally from the channel in one direction only, whereafter the rod can be removed radially out of the channel. A nut threaded onto the rod prevents the device from sliding in one direction along the rod and simultaneously obturates the outer end of the bore to prevent the sleeve from leaving the bore, provided that the device is adjacent the nut. This implant has the disadvantage that removal of a nut from among the nuts on the rod involves the disconnection of the hook-like devices between that nut and one end of the rod, as does insertion of an extra nut among the nuts on the rod. Moreover, the need to thread the nut along the rod results in the device being rather slow to assemble and can result in damage to soft tissue if carried out in situ. Another disadvantage is that, with the apparatus in situ, the hook part is not fixed by the apparatus against rotation about the rod and is fixed by the apparatus against movement along the rod in one direction only. Another known implant, but which is for use anteriorly, comprises a cable and several devices which are fixable in a substantially permanent manner to the cable at intervals therealong. These devices resemble bone screws in order that they may be able to be screwed into respective vertebrae. A third known implant, also for use anteriorly only, comprises a threaded rod, whilst its bone-screw-like devices incorporate rotatable nuts receiving the threaded rod and adjustable therealong by rotation, in order to allow in situ adjustment of the vertebral levels towards and away from each other.
Described with reference to FIG. 8, for example, of European Patent Specification No. 0128058 is a hook-like device consisting of three separate parts, namely a hook part provided with a U-shaped channel for receiving a rod; a sleeve part formed with a conical external surface arranged to be pressed into a correspondingly shaped internal surface of the channel from one end of the channel to grip the rod by a wedge effect, the sleeve part carrying a set screw for clamping the sleeve part to the rod and also including teeth for engaging in stabilization notches in the said one end of the channel; and a ring part arranged to abut the other end of the channel, carrying a set screw for clamping the ring part to the rod and also including teeth for engaging in stabilization notches in the other end of the channel. Not only is this apparatus somewhat complicated to assemble, with the added difficulty that the surgeon has to ensure for each device that the wedge effect, the tightening of both set screws and the engaging of both sets of teeth in their notches have all been correctly achieved, but, if with the apparatus in situ, he decides to add or remove a device at a particular vertebra, all of the other devices between that vertebra and the nearer end of the rod have to be dismantled to permit addition or removal of the device.